Microsoft has held out a glimmer of hope to Sidekick users, saying that it may be able to recover some data previously believed lost in a massive server failure.

Microsoft late Monday held out a glimmer of hope to Sidekick users, saying that it may be able to recover some data previously believed lost in a massive server failure.

"Recent efforts indicate the prospects of recovering some lost content may now be possible," a Microsoft spokesman said in a statement that was duplicated on T-Mobile's support site. "We will continue to keep you updated on this front; we know how important this is to you."

The news came two days after Microsoft and T-Mobile confirmed that a server failure "almost certainly" meant that users' data had been lost. In a joint message at the time, the companies said that although engineers were working on the problem, "the likelihood of a successful outcome is extremely low."

The outage sparked users to post thousands of messages on T-Mobile's support forums, where most customers raged at the loss, calling it "inexcusable" and beating the drum for a class-action lawsuit.

On Monday, hints surfaced that Microsoft might have made progress in restoring the lost data, as some users said that personal data had reappeared on their phones.

Sidekicks use the servers run by Microsoft subsidiary Danger to synchronise the smartphone's content, including contacts, appointments and photos, with a cloud-based storage service. When the servers went down and data on them was lost, that same data was then deleted from Sidekicks whose users had removed the battery in an attempt to reset the device, or had let the phone's battery completely drain.

Some reports had linked the server failure to an upgrade of Danger's storage area network (SAN). Microsoft, however, declined to confirm those reports.

T-Mobile promised customers $100 for their troubles, but with significant caveats.

"In the event certain customers have experienced a significant and permanent loss of personal content, T-Mobile will be sending these customers a $100 customer appreciation card," the mobile carrier said.

"This will be in addition to the free month of data service that already went to Sidekick data customers. This card can be used towards T-Mobile products and services, or a customer's T-Mobile bill."

Affected customers will be automatically notified within the next two weeks, T-Mobile added. "We however remain hopeful that for the majority of our customers, personal content can be recovered."

Also on Monday, Microsoft provided some additional information about what happened at Danger, the Sidekick software and services developer it acquired in 2008.

"A server failure at Microsoft/Danger caused an outage that affected the applications and services available on the Sidekick devices," a Microsoft spokesman said Monday afternoon. The failure, the spokesman added, "impacted both the main and backup databases."

But Microsoft also made a point to distance its own cloud-based plans from the disaster. "The Sidekick runs on Danger's proprietary service that Microsoft inherited when it acquired Danger in 2008," said Microsoft's spokesman.

"The Danger service is built on a mix of Danger-created technologies and third-party technologies. Microsoft's other cloud computing projects are totally separate from the Danger Service and do not rely on the Danger Service technology."

T-Mobile, meanwhile, removed Sidekick from its online store Monday. As of 2am Eastern today, the Sidekick was still listed as "temporarily out of stock."